1. Field of the Invention
The present invention relates to a semiconductor sensor device having a correction circuit and, more particularly, to the structure a read circuit having a correction circuit for correcting or compensating variations of the sensor array.
2. Description of the Related Art
A read circuit having a bias circuit is used in a semiconductor sensor device having a sensor array such as an infrared-ray imaging device. Patent Publication JP-A-11-150683 describes a conventional imaging device shown in FIG. 1. The imaging device includes an array of thermocouples (thermoelectric sensor) 1101, an array of switches 1100 each connected serially with a corresponding one of the thermocouples 1101, and a read circuit including an emitter follower having a PNP transistor 1104 and an NPN transistor 1102, and a resistor 1103, for detecting the resistances of the thermocouples 1101.
The read circuit of the imaging device further includes a FPN correction current source 1113, for conducting a correction current from a first node connecting the collectors of the transistors 1104 and 1102 together, an integration capacitor 1105 connected across the first node and the ground, a reset switch 1106 for connecting the voltage source Vr and the first node, source follower transistors 1107 and 1108 receiving a gate signal from the first node, a sample/hold circuit including a switch 1109 and a capacitor 1110 for receiving a signal from the source follower transistors 1107 and 1108, and output source follower transistors 1111 and 1112 for receiving a signal from the sample/hold circuit to output the pixel, data read from the thermocouples 1101.
The thermocouple 1101 is a titanium bolometer which changes the electric resistance thereof depending on the temperature. The titan bolometer is sensitive to an incident infrared ray. When a bias voltage VB1 is applied to NPN transistor 1102, a voltage of (VB1−VBE) is applied across the titan bolometer, wherein VBE is the base-emitter voltage of NPN transistor 1102. Thus, a collector current of IC1=(VB1−VBE)/RB1 flows toward NPN transistor 1102, given RB1 being the resistance of the titan bolometer 1101.
The resistance RB2 of resistor 1103 is used as a reference resistor for the titan bolometer 1101. A bias voltage of VB2 applied to PNP transistor 1104 allows a collector current of IC2=(VB2−VBE)/RB2 to flow toward NPN transistor 1104.
While an infrared ray is not incident onto the bolometer, the base voltage of transistor 1104 is controlled so that both the currents IC1 and IC2 are balanced to each other. In this case, a current hardly flows toward the integration, capacitor 1105. When an infrared ray is incident onto the bolometer 1101, the temperature of a diaphragm which is thermally isolated rises, whereby the resistance, of the titan bolometer 1101 mounted on the diaphragm is changed. The change of the resistance changes the current IC1, whereas the current IC2 does not change because the resistance of the resistor 1103 formed by diffusion does not change. The change of IC1 generates a current difference ΔI wherein ΔI=IC2−IC1, which stores electric charge on the integration capacitor 105. The current difference ΔI includes a signal component from the titan bolometer 1101 and a noise component of the bias current although a larger component of the bias current is removed therefrom.
In the imaging device of FIG. 1, if the variations of the resistance RB1 between the pixels are large, the FPN (fixed pattern noise) correction circuit which includes elements 1106 to 1108 allows a specified correction current Ifpn to flow for each of the pixels. That is, if resistance RB1 of the titan bolometer is larger than the standard resistance, the current IC1 flowing through the bolometer is smaller, whereby the current difference ΔI=IC2−IC1 assumes a larger value. The correction current Ifpn reduces the effective current for IC2 which is equal to IC2−Ifpn, whereby the difference of resistance RB1 is compensated by the correction current Ifpn.
The signal charge stored on the integration capacitor 1105 is applied to the source follower transistors 1107 and 1108, which convert the high impedance signal of the first node to a lower impedance signal. The sample/hold circuit samples and holds a time series signal, and delivers the sampled signals through the output source follower transistors 1111 and 1112 as S/Hout signals. The switch 1109 is implemented as a transfer gate including an nMOSFET and a pMOSFET connected in parallel.
In the conventional imaging device as described above, the FPN correction current Ifpn can correct the resistance variations if the thermocouple has a larger resistance RB1 compared to the standard resistance. However, if the thermocouple has a lower resistance RB1 compared to the standard resistance, or if the thermocouple has an excessively larger resistance which cannot be compensated by the full-scale correction current Ifpn, canceling current for canceling the variations must be manually adjusted, if the imaging device has an adjustment element for adjusting the canceling current. The manual adjustment of the canceling current is a time-consuming work, however. If the imaging device has no such an adjustment element for the canceling current, the compensation of the smaller resistance or the excessively larger resistance cannot be compensated, whereby it is difficult to raise the amplification factor for the detected signal.
In general, the imaging device having a plurality of pixels involves variations between pixel data. This is noticeable in the case of an infrared-ray imaging device or an amplification type imaging device. The causes of the variations between the pixels include variations of the detectors such as bolometers, and the threshold voltage or parasitic capacitance of the amplifying circuit. In the case of the infrared-ray imaging device having bolometers, the resistance of the bolometer varies within several percents to several tens of percents due to the variations of the thickness or specific resistance of the bolometer film, and variations in the dimensions of the bolometer after the patterning.
Those variations impede the imaging device from reading an accurate signal. For example, the signal difference of the bolometer is as low as 1 m° C. in terms of signal temperature for a temperature difference of 1° C. of an object. In this case, the resistance change of the bolometer is as low as around 0.001 percent, assuming that the bolometer has a resistance temperature coefficient of 1%/° C. In such a case, the small signal is preferably amplified in the imaging device; however, the larger variations limit the dynamic range of the amplifier whereby the amplification factor is restricted.